Filter aids are taken to mean additives which are used in solid-liquid separation processes in order to ensure deposition of the solids with simultaneously sufficient permeability of the resultant filter cake by formation of a porous precoat layer on the actual filter medium and/or by incorporation into the filter cake structure.
As filter aids, use is made not only of inorganic substances such as, for example, kieselguhr, perlite or aluminum oxides, but also synthetic polymers. Which filter aids are used in individual cases depends also on the field of application. Semisynthetic filter aids within the meaning of the invention means modified natural substances which have been chemically or physically changed in their properties, such as modified cellulose for example.
In the filtration of beer, kieselguhr is one of the most frequently used filter aids.
For economic reasons it is advantageous when the filter aid is regenerable. Regeneration over a plurality of filtration-regeneration cycles is particularly advantageous.
WO 02/32544 (US 2004 094 486) describes coextrudates of polystyrene and water-insoluble polyvinylpyrrolidone and use thereof as regenerable filter aids, wherein, however, the regenerability is mentioned only in quite general terms.
WO 03/084639 (US 2005 014 5579) describes coextrudates made of thermoplastic polymers except for polystyrene, and water-insoluble polyvinylpyrrolidone and use thereof as regenerable filter aids, wherein, however, the regenerability is mentioned only in quite general terms.
WO 92/11085 (U.S. Pat. No. 5,484,620) describes filter aids based on agglomerates of crosslinked polyvinylpyrrolidone and fibrous thermoplastic polymers such as, for example, polyethylenes or polyamides, and use thereof as filter aids. Reference is made in quite general terms to the fact that the filter aids are regenerable.
EP 611 249 describes a method of regenerating a filter aid by adding enzymes and sodium hydroxide solution.
EP 253 233 describes the regeneration of kieselguhr only by means of sodium hydroxide solution, as result of which the polysaccharides of the microorganisms (beta-glucans) are not destroyed.
DE 196 25 481 describes the regeneration of kieselguhr in the presence of a mixture of nonionic and anionic surfactants and proteolytic enzymes.
DE 196 52 499 describes the regeneration of filter aids, wherein the filter aids are removed from the filter elements and treated in a separate vessel with enzyme solutions, weak alkalis and weak acids, and also with an oxidizing agent.
WO 03/008067 (US 2005 005 1502) describes the regeneration of the filter aids, wherein, on the intact filter cake, first rinsing with alkaline solution proceeds, and subsequently likewise on the intact filter cake, neutralization of the pH by rinsing with acid.
WO 96/35497 (U.S. Pat. No. 6,117,459) discloses the regeneration of filter aids made of synthetic polymers such as polyamide filter aids by washing with alkaline solution and washing with an enzymatic compound, wherein the treatment proceeds in situ or in the filter unit on the intact filter cake.
EP 879 629 describes an alkaline solution treatment of a suspended filter aid, and if appropriate an enzyme treatment.
GTM 3/2006 (Verfahrenstechnik Filtration, pages 44-46) describes regeneration of kieselguhr by means of NaOH and nitric acid by means of enzymes (beta-glucanase, protease) or a combination of the two methods, wherein 70% of the kieselguhr could be reused.
It has been shown, however, that the previously known regeneration methods for the filter aids to be treated according to the invention do not give satisfactory results. Depending on the regeneration procedure, virtually complete use of the filter aid over a plurality of filtration-regeneration cycles is impossible. To achieve a plurality of cycles, currently, per cycle, about 30% by weight of fresh filter aid must be added (Verfahrenstechnik Filtration, pages 44-46).